Bubble traps used in blood lines for hemodialysis or the like conventionally comprise a typically rigid or semi-rigid tube in which a blood inlet is provided to convey blood into the top of the chamber, while a blood outlet draws blood from the bottom of the chamber. Bubbles then are given the opportunity to rise to the top of the chamber so that the blood in the bottom of the chamber, which is withdrawn to pass through another portion of the blood set, is relatively free of bubbles, since they migrate to the top of the chamber.
See also Utterberg U.S. Pat. Nos. 5,328,461 and 5,520,640 as other examples of bubble traps for blood lines known to the prior art.
Typically, such bubble traps are higher than they are wide, to provide a deep, vertical chamber for the blood so that bubbles are kept away from the bottom of the chamber, from which the blood is being withdrawn. Typically, the prior art bubble traps have chambers with a vertical height that is more than twice their width. The height of the chambers of the prior art, coupled with the buoyancy of the incoming bubbles, is intended to counteract the downward bulk fluid flow of blood in the chamber toward the bottom outlet.
The inlets of the prior art blood chambers are variably positioned, the idea being that the blood entering through such inlets, and the bubbles contained in the blood, will initially stay in an upper portion of the chamber so that the bubbles have time to migrate upwardly through a liquid level to a gas space at the top of the chamber. Some inlets are vertically oriented, extending downwardly from the top of the chamber. Because of the height of the chamber, inflowing blood stops moving downwardly before the bubbles contained in it can be caught in the outlet flow. Other inlets of the prior art are vertically oriented in the bottom of the chamber, to propel the inlet blood upwardly toward the chamber top. Other inlets are horizontally oriented in the side of the chamber, so that the inlet flow must horizontally cross the downward flow of the bulk blood in the chamber, moving to an opposite sidewall where it is turned upwardly. This raises the possibility of bubbles being entrained in the downward flow before they are turned upwardly to reach the intended air space.
The bubble trapping principles of the prior art are effective with large, buoyant bubbles, typically having a volume greater than 50 microliters, and at relatively low blood flow rates of less than 300 ml. per minute. Blood chambers for trapping bubbles typically have volumes of about 15-25 ml. The buoyancy of the bubbles urges them to the surface at a velocity greater than the downward velocity of the bulk flow of the fluid in the bubble trap.
However, such bubble traps are increasingly ineffective as bubbles get smaller, and/or as flow rates increase. Modern dialysis techniques often require blood flow rates exceeding 450 ml. per minute, which raises the risk that bubbles can get through bubble traps of the prior art.
To accommodate such higher flows, the volumes of some designs of prior art bubble traps have been increased. However, this is distinctly undesirable, since that increases the priming volume of the set. It is highly desirable to keep the priming volume of any blood set low, since it is important to minimize the amount of blood removed from a patient at any one time during a blood treatment procedure such as dialysis.
Furthermore, another problem of prior art bubble traps, particularly those with the upwardly oriented inlets, is that they may require a flow diverter, to prevent blood at high flow rate from bursting through the blood-gas interface in a geyser-like action, which causes foaming of the blood and consequent clotting in the chamber. A typical blood flow diverter comprises an indentation in the wall of the bubble trap, to force the upwardly moving stream of inlet blood into a more horizontal flow, to prevent such geyser-like action. However, the diverter itself is not deemed desirable, and may result in an increased number of bubbles to be driven down toward the bottom outlet and thus to pass out of the bubble trap, contrary to that which was intended.
In accordance with this invention, solutions to the above technical problems are provided, resulting in an improved flow-through bubble trap for blood lines or the like, which is capable of processing blood at high flow rates of 450 ml. per minute and greater, while still retaining a low chamber interior volume.
In Schnell and Utterberg application Ser. No. 08/755,806, filed Nov. 26, 1996, now abandoned, wide bubble traps are disclosed in which the width of the bubble trapping chamber is preferably wider that the height of the chamber. The fluid inlet and fluid outlet to these chambers are then laterally spaced from each other to provide a fluid flow pattern which typically is substantially horizontal in nature, with less of a vertical flow component than in the prior art. This has been found to facilitate the migration of bubbles upwardly to the top of the chamber.